Terahertz radiation, sometimes called “T-rays,” remains a relatively unexplored region of the electromagnetic spectrum. Terahertz radiation lies in the frequency band between about 1011 Hertz (Hz) and about 5×1013 Hz and therefore between microwaves and infrared light. Terahertz radiation is important, because it has potential use in many military, security, commercial, biomedical, pharmaceutical and scientific research applications. Terahertz radiation can penetrate most solid substance and so behave like X-rays. Unlike X-rays however, terahertz radiation is non-ionizing and thus substantially safer to use. Terahertz radiation also can produce images of higher resolution than X-rays. Because of terahertz radiation's ability to penetrate fabrics and plastics, it can be used in surveillance, including security screening, to uncover concealed weapons on a person. This is highly useful, because many materials of interest, such as plastic explosives, exhibit unique spectral “fingerprints” in the terahertz range. This offers the possibility of combining spectral identification with imaging. Terahertz radiation can also be used to characterize semiconductors more accurately and widen wireless communication frequency bands.
Because they can penetrate several centimeters into human flesh, terahertz radiation can detect tumors far better than today's mammograms and can detect skin cancer before it appears as lesions on the skin. Coupled with tomography algorithms, terahertz radiation can be used to create a 3-D map of the human body that has a far higher resolution than one produced by nuclear magnetic resonance (NMR), paving the way for a host of diseases to be detected earlier and more effectively.
A heavy demand for terahertz radiation also exists in the communications industry. For example, a terahertz-frequency heterodyne receiver can dramatically increase the available bandwidth in wavelength-division-multiplexed (WDM) communication systems.
However, while the benefits of having terahertz radiation have been understood for over a decade, having a powerful source of terahertz radiation has eluded technologists so far. Currently, two basic techniques for generating terahertz radiation are used: photoconduction and nonlinear optical frequency conversion. Electrically biased high-speed photoconductors may be used as transient current sources for radiating antennas, including dipoles, resonant dipoles, transmission lines, tapered antennas and large-aperture photoconducting antennas. U.S. Pat. Nos. 6,144,679, 6,697,186 and 5,543,960 teach various ways in which second- or higher-order nonlinear optical effects in unbiased materials may be employed to generate terahertz radiation.
No matter which technique is chosen, the terahertz power generated is usually on the order of microwatts, even with many watts of input power. The inability to generate significant terahertz power places most real-world applications out of reach. In addition, substantial cooling is required with both photoconduction and nonlinear optical frequency conversion due to their low efficiency.
Given the above, what is needed in the art is a better technique for generating terahertz radiation. Further, because microwave radiation also finds great use in a host of applications, what is needed in the art is a better technique for generating microwave radiation. More particularly, what is needed in the art is a better technique for generating significant terahertz or microwave power.